Polyurethane foams are widely used as materials from which articles such as mattresses, seat cushions, and thermal insulators are fabricated. Such polymeric foam materials are ordinarily manufactured by a casting process in which a mixture of liquid polyurethane-foam-generating reactants are deposited in a mold. As used herein, the term "mold" includes both stationary molds for batch casting and translating or otherwise moveable molds for continuous casting. Evolution of a gas causes the reactants to foam. For some foam formulations, the reactants themselves react to evolve sufficient gas; in others, a blowing agent is mixed with the reactants to provide gas evolution. Continued gas evolution causes the foam to expand to fill the mold. The foam becomes increasingly viscous as the reactants polymerize, ultimately curing into a polyurethane foam casting shaped by the mold.
Slabs of polyurethane foam approximately rectangular and round in cross section are conventionally cast in a translating channel-shaped mold. Such molds typically include a belt conveyor forming the bottom of the mold and a pair of spaced-apart, opposing side walls, which can be fixed or translatable at the speed of the conveyor. The mold sides and bottom are generally lined with one or more sheets of flexible-web such as kraft paper or polyethylene film. The sheets of mold liner are ordinarily withdrawn from rolls and continuously translated along the mold channel at the same speed as the belt of the conveyor. A liquid foam-generating reaction mixture is deposited on the mold bottom in a zig-zag pattern from a nozzle positioned above the mold which is reciprocated back and forth across the width of the mold. Typically, as the foam expands, the reaction mixture will merge into a uniform slab of foam.
If fresh reaction mixture is deposited on top of foam generated from previously deposited reactants, the resulting cured foam will have an uneven surface and nonuniform density, which is undesirable for most applications. By continuously translating the mold liner, the reaction mixture is continuously carried away from the pouring area below the pouring nozzle, which reduces the tendency for fresh reaction mixture to cover previously deposited mixture.
Propitious selection of conveyor speed can prevent production of undesirable foam products. A range of speeds can be established for a particular reaction mixture formulation. Minimum speed is achieved when liquid reaction mixture is evenly distributed on the bottom of the mold and does not flow in a direction opposite to that of the mold and conveyor. Selection of an appropriate speed requires consideration of the chemical reaction occuring subsequent to the depositing of liquid mixture in the mold. During residence in the mold, the liquid mixture foams and cures. Because economy necessitates maximum product height, lower speeds are preferred during the foaming portion of the reaction to attain such heights.
To reduce further the tendency of the liquid reactants to flow back under the pouring nozzle and to assist the "zig-zags" of reaction mixture to merge uniformly, it is customary to incline a pouring board, the surface under the nozzle, from horizontal so that the bottom liner slopes downward in the direction of translation. The maximum angle of inclination is different for different foam formulations, such as polyester polyurethane foams.
Also, problems arise if the mold bottom slopes downward along its entire length. Conventional continuous slab molds are quite long, typically in excess of 60 feet, to provide for integrity of the foam. Building a translatable mold of this length inclined from horizontal is significantly more expensive than building a translatable mold of the same length which is horizontal, because, for example, the building housing and the super structure supporting the inclined mold would require a higher investment. Moreover, it is especially expensive to provide for changing the angle of inclination of the entire mold to compensate for differing viscosities among the various foam formulations. Thus some continuous slab molds have horizontal belt conveyors for most of the length of the mold bottom, but have relatively short inclined and adjustable pouring boards located beneath the pouring nozzles. The expansion and rise of the foam generally takes place on the sloping pouring board.
A second reason for providing a pouring board which makes an angle with respect to the belt conveyor concerns the cross-sectional shape of the slab cast the the mold. As the foam expands and rises in the mold, it encounters the sides of the mold. If the mold-side liners are being translated substantially parallel to the mold bottom, the expanding foam experiences a shear force which resists its rise along the sides. This shear force results in a rounding of the top of the rectangular slab to form a crown or crest of convex shape, much like a loaf of bread. For most applications such rounded portions are unusable and must be discarded as scrap. Thus, the more nearly rectangular the cross section of the slab, i.e., the flatter the top, the more economical is the casting process. U.S. Pat. No. 3,325,823 describes one method known and used commercially for making flat top blocks of polyurethane foam.
If, over the length the foam travels as it expands, the mold bottom liner and the two mold side liners are translated, not in parallel, but at an angle with respect to one another, the mold side liner can have a velocity component relative to the mold bottom in the direction of the expansion of the foam which can compensate for the shear force which resists the rise of the foam. Guiding the mold-bottom liner across an inclined pouring board, which is located between the side walls of a slab mold and intersects the mold-bottom conveyor at an angle, can provide such a compensating velocity component when the foam expansion is carried out over the length of the pouring board and mold-side liners are translated parallel to the mold-bottom conveyor. The angle of intersection which ordinarily leads to polyurethane foam slabs having the most nearly rectangular cross sections is about 10.degree. for typical foam formulations and production conditions. Unfortunately, if the pouring board is sloped about 10.degree. from horizontal, freshly deposited reaction mixture tends to flow forward and under already-deposited reaction mixture, as discussed above, leading to foam slabs of nonuniform density or otherwise imperfect.
Although it is possible to construct a continuous slab mold with a pouring board inclined from horizontal by an angle of 4.5.degree. and intersecting the belt conveyor at 10.degree., the belt conveyor in such a case is normally inclined upward by an angle of 5.5.degree.. See, for example, U.S. Pat. No. 3,325,823. As noted above, however, inclined translatable molds are more expensive than comparable horizontal molds.
U.S. Pat. No. 3,786,122 discloses a process for producing polyurethane foam slabs which employs a horizontal, channel-shaped mold having at its forward end an inclined "fall plate" which makes an angle of significantly greater than 4.5.degree. from horizontal. The problem of reaction mixture flowing down the inclined fall plate is obviated by prereacting the reaction mixture prior to introducing it onto the fall plate. The prereacting step is carried out in a trough which opens onto the upper edge of the fall plate. Liquid foam reactants are introduced onto the bottom of the trough and the foam which is generated is allowed to expand upwards in the trough and spill over onto the fall plate. The foam continues to expand as it is carried down along the fall plate by a translating bottom sheet. Because the prefoamed reaction mixture exiting the trough is more viscous than the initial liquid reaction mixture, the fall plate can be inclined at a greater angle from horizontal than a pouring board in a conventional polyurethane-foam slab mold.
An additional result of introducing prefoamed reaction mixture into the mold is that relatively high foam slabs can be produced as compared with conventional processes. The height to which foam rises can be thought of as being divided into two components, a first component is the result of the expansion of the foam below a horizontal plane passing through the point at which the reactants begin to foam and is determined by the decline and length of the pouring board, and a second component is the result of the rise of the foam above the horizontal plane.
Economies result from producing high slabs because, the thicker the foam slab, the less is the loss from discarding the skin or rind which generally coats polyurethane foam castings. With a conventional slab mold, if the rate of introduction of reaction mixture is kept constant and the rate of translation of the mold liner is reduced, the height of the foam slab tends to increase because more foam-generating reactant is deposited per unit length. However, because the rate of gas evolution remains essentially constant, the rising of the foam takes place over a linear distance, in addition to rising to a greater height, which gives the rising foam a steeper slope. If the rate of translation is slowed sufficiently, this slope becomes so steep that the expanding foam, particularly the youngest and most fluid portion, becomes unstable and tends to slip and shift, which results in cracks and other imperfections in the cured foam.
This problem of instability of rising foam is reduced in the process of U.S. Pat. No. 3,786,122 by introducing into the translating mold prefoamed reaction mixture which is sufficiently viscous as to be able to sustain a relatively steep slope of the pouring board as it completes its expansion. Thus the first component which determines the height of the foam can be increased. In addition to permitting higher foam slabs to be cast by reducing the translation speed of the mold liner, this process permits the use of slab molds shorter than those of conventional processes, because the slab moves a shorter distance during the curing time.
In practice, however, the process of U.S. Pat. No. 3,786,122 suffers from a number of drawbacks. The prefoamed reaction mixture introduced into the mold must be quite fluid, because the foaming mixture rising in the trough must, by gravity flow, spill over a weir structure and onto the fall plate of the mold. Thus prefoamed reactants which are too viscous to flow freely such as the polyester type cannot normally be used. This limits the height of slabs which can be obtained by the process.